1. Field of the Invention
This invention relates to performance of computer systems and more particularly to performance associated with cache memory.
2. Description of the Related Art
A processing node in a computer system may be placed in any of multiple performance states (or operational states) Pn, where the particular performance state (or P-state) is characterized by an associated voltage and frequency. The Advanced Configuration and Power Interface (ACPI) Specification defines the active state C0 and various low power states C1-C4. Some processors define additional low power states. Associated with the C0 state are various performance states. One factor for determining the appropriate performance state of a node is its utilization. Utilization is the ratio of the time spent by the processing node in the active (execution) state to the overall time interval over which the execution time was tracked or measured. For example, if the overall time interval was 10 milliseconds (ms) and the processor node spent 6 ms in the active (C0) state, then the utilization of the processor node is 6/10=60%. The processor node spends the remaining 4 ms in the idle (non-C0) state where code execution is suspended. A higher node utilization triggers the selection of a higher performance state P higher voltage and frequency to better address performance efficiency requirements, as measured by performance per watt. Normally the decision to transition the processing node between performance states is made by either the operating system (OS), or high-level software, a driver, or some hardware controller. For example, if the processing node runs at a low performance state resulting in longer code execution time, the system perceives the need for a higher utilization and triggers software or hardware to transition the processing node to a higher performance state where it can complete code execution faster and spend more time in the idle state. That allows increased power savings from an overall better performance per watt. While using utilization as a trigger can provide increased performance per watt in some situations, it fails to address some issues associated with better performance per watt or preventing its degradation.